1. Field of the Invention
The present invention relates to amalgam compositions for restorative dental repair.
2. Description of the Prior Art
Dental amalgam has been available to the dental profession for well over a century and it is used extensively for intracoronal and extracoronal restorations. Amalgam is highly durable and the strength and occlusal wear characteristics of alternative materials such as composite resins are generally compared to that of amalgam. However, amalgam does not adhere to tooth structure and the dentist must take great care to prepare the tooth cavity with dovetails and various cutout grooves which in effect mechanically lock the amalgam into the cavity. Such required preparation by the dentist results in the need to excavate more tooth structure than would otherwise be necessary if the amalgam were adhesive. This of course weakens the tooth. Additionally the problem of microleakage at the interface of the amalgam and cavity wall tends to occur for a period after the amalgam has been placed into the cavity. Microleakage allows penetration of bacteria, soluble salts and saliva into any space between the amalgam restoration and cavity walls. This can lead to inflammation and pulp irritation which in turn can cause other complications. The penetration of bacteria into spaces between the amalgam and cavity wall can demineralize the cavity walls and lead to formation of recurrent caries. Corrosion of amalgam can cause one of the amalgam alloy metals, for example tin, to deposit along the amalgam and cavity wall interface. This usually leads to tooth discoloration along the interface and can noticeably detract from the esthetic appearance of tooth and restoration. An adhesive seal between amalgam restoration and cavity walls could prevent microleakage. An adhesive amalgam could significantly reduce the amount of tooth the dentist needs to excavate in order to prepare the cavity for restoration. An adhesive amalgam could, impart significantly more strength to the filled tooth structure.
The concept of making an adhesive amalgam is thus attractive from many standpoints. Unfortunately the nature of amalgam and teeth makes it very difficult to adhere amalgam to tooth structure.
U.S. Pat. No. 3,513,123 (Saffir) discloses an epoxy liquid resin composition which is added to amalgam in and effort to make the amalgam adhere to tooth structure. This reference discloses use of an epoxy liquid resin additive consisting of a glycidyl ether type epoxy resin containing a polyamine hardening agent.
Various references disclose mixtures of amalgam with various additives to impart improved mechanical characteristics. For example U.S. Pat. No. 4,859,412 (Grell) discloses the addition of ceramic or glass powder to alloy powder, which when amalgamated with mercury, produces an amalgam with improved mechanical strength properties. Japanese patent publication 55-22545 discloses use of glass additives which can be blended with amalgam during the amalgamation of alloy powder with mercury. The amalgam modified with glass additives is alleged to impart improved compressive strength properties.
U.S. Pat. No. 2,991,176 (Clancy) discloses mixtures of silica powder, alumina and other materials with amalgam alloy powder. The amalgam alloy powder and other materials are milled to form particles wherein these materials are encapsulated by the amalgam alloy powder. When the encapsulated particles are amalgamated with mercury, a modified amalgam is formed and is said to have reduced thermal coefficient of expansion which in turn reduces the chance for microleakage.
U.S. Pat. Nos. 4,255,192 (Burns), 4,684,347 (Palaghias) and 3,676,112 (Muhler) disclose modified amalgams which utilize various additives or treatment of the amalgam alloy powder to impart varying physical or cariostatic properties to the amalgam.
U.S. Pat. No. 4,064,629 (Stoner), discloses a method for applying amalgam restorations which involves precoating the surfaces of a cavity within a carious tooth with a layer of an "adhesive-metal" lining composition. The metal of the lining composition is amalgamated by diffusion of the mercury from the subsequently applied conventional dental amalgam filling. The "adhesive-metal" lining composition is said to improve corrosion resistance of the dental amalgam filling and also promotes bonding between the amalgam restoration and the cavity surfaces. Other references which disclose precoating the surfaces of a tooth cavity with an adhesive coating said to adhere to conventional amalgam are, for example, U.S. Pat. Nos. 4,001,483 (Lee) and 3,574,943 (Nicholson).
In recent years several adhesive products which claim to make amalgam adhesive to tooth structure have been made available to dental clinicians. (The term "tooth structure" as used hereinafter shall be interpreted to include either or both dentin and enamel, optionally precoated with liner or base.) One such product is sold in a kit form under the trademark "AMALGAMBOND" available from Parkell Co. The "AMALGAMBOND" product is a liquid adhesive resin which is coated directly onto tooth structure. The application and curing procedure are cumbersome and involve a number of steps. The curing procedure also requires use of an air-sensitive catalyst which if dropped on flammable paper causes smoldering. The active ingredients in the adhesive are 4-META (4-methacryloxyethyl trimellitic anhydride) and TBB (tri-n butyl borane). Other products which similarly involve coating a specific curable resin directly onto tooth structure to make amalgam adhere are available under the trademarks "PANAVIA" Dental Adhesive from Kuraray Company and "SUPERBOND" Adhesive from Sun Medical Co., Ltd., Kyoto, Japan. These latter products also are difficult to employ, since there are a number of required preparatory steps for their application and curing.
Literature articles which disclose bonding of amalgam to tooth structure by precoating the tooth with adhesive resin include M. Staninec and M. Holt, Journal of Prosthetic Dentistry (1988), Vol. 59, p. 397-402, A. Lacey and M. Staninec, Quintessence International (1989), vol. 20, p. 521-524, and Y. Torii, et al. Operative Dentistry (1989), Vol. 14, p. 142-148. The above listed articles report improved adhesive tensile strength between amalgam and coated tooth structure but do not report adhesive shear bond strength of the amalgam.